I'm afraid the only hordes that are always available are: Peasants, bowman, swordsman, maybe priests and guardsman (i've never cared about them in the late game, so i haven't checked), horsemen, knights (which replace the horde of peasants), and somewhere in the freedom isles, you will probably find a horde of sea dogs and pirates.
The capital of Ellinia will have a horde of some unit after freeing the queen's soul, not necessarily sprites. I have seen sprites, dryads and hunters, but not all in the same game. Other random stores occasionally have hordes, and may always have a horde (eg, in two games, i've found a horde of druids in the store by the dock in 1000 rivers (ie, where you get the flower quest), but i never checked in the other game, so i'm not sure if it is certain to be there.)
On another note, I find the thought of hordes to be HILARIOUS! If, when you open up the purchase screen for that unit, you right/left (i forget which) click on the picture of the unit, you can see how many units are in the horde. That horde of druid i found had over 98,000,000 druids in it, in this small house. I don't know where he puts them all
